1. Field of the Invention
Aspects of the present invention relate to a synchronous rectifier circuit and a multi-output power supply device using the same, and more particularly, to a synchronous rectifier circuit capable of controlling an output voltage and a multi-output power supply device using the same.
2. Description of the Related Art
Image forming apparatuses such as computers, printers, and photocopiers, and devices such as monitors and communication terminals require a high-efficiency power supply system that can supply stable power while having a simple and compact structure. Such high-efficiency power supply system generally employs a power supply device with multiple outputs that produces multiple voltage outputs using a single transformer with multiple-windings.
FIG. 1 illustrates a typical multi-output power supply device using a synchronous rectifier circuit, and FIG. 2 illustrates a typical multi-output power supply device using a secondary side post regulator (SSPR) circuit. The circuits illustrated in FIGS. 1 and 2 are also referred to as a flyback converter, which is a kind of a DC/DC converter.
Referring to FIG. 1, the typical multi-output power supply device includes a transformer T having a primary coil L1 and two secondary coils, that is, first and second coils L2 and L3, which respectively have predetermined turn ratios with the primary coil L1. The primary coil L1 of a primary side of the transformer T is coupled to a primary circuit 10. The first coil L2 of a secondary side of the transformer T is coupled to a first output circuit 20. The second coil L3 of the secondary side of the transformer T is coupled to a second output circuit 30, which is a synchronous rectifier circuit.
The primary circuit 10 includes a control switch S serially connected between the primary coil L1 of the transformer T and a ground terminal. As such, the control switch S can switch an input voltage Vi′ in response to a control signal received from a primary switching controller 15 so as to control an energy charging or transferring operation of the transformer T.
The first output circuit 20 includes a rectifier 21 for rectifying a current transferred from the transformer T, and the rectifier 21 comprises a diode D1 and a capacitor C1 connected in series to the first coil L2 of the secondary side of the transformer T. An output terminal of the rectifier 21 is formed across the capacitor C1, and thus, an external load can be connected in parallel with the capacitor C1.
The second output circuit 30 comprises a semiconductor switch Q and a capacitor C2 connected in series to the second coil L3 of the secondary side of the transformer T. The semiconductor switch Q repeatedly conducts (allows flow of current) and non-conducts (prevents flow of current) according to a synchronous rectification (SR) control signal used to perform synchronous rectification. Another output terminal is formed across the capacitor C2, and thus, an external load can be connected in parallel with the capacitor C2.
The primary switching controller 15 applies a control signal for controlling a duty rate of the control switch S. Such a control signal is generated by feeding back of an output voltage Vo1′ of the first output circuit 20. As such, output voltages of the typical multi-output power supply device can be controlled by controlling the operation of the control switch S.
However, the output voltages of the typical multi-output power supply device illustrated in FIG. 1 is controlled through a single controller, that is, the primary switching controller 15. As such, cross regulation of the first and second output circuits 20 and 30 becomes a problem. That is, the output voltages Vo1′ and Vo2′ of the first and second output circuits 20 and 30, respectively, cannot be independently controlled. To solve this problem, the typical multi-output power supply device using a secondary side post regulator (SSPR) circuit, as illustrated in FIG. 2, was proposed.
Referring to FIG. 2, the typical multi-output power supply device includes the SSPR circuit as the second output circuit 40 comprising a diode D2, a semiconductor switch Q′, and a capacitor C2 connected in series to the second coil L3 of the secondary side of the transformer T. The diode D2 and the capacitor C2 perform a rectification operation and the semiconductor switch Q′ controls the output voltage Vo2′ in response to an output control signal (SSPR control signal) generated by feeding back of the output voltage of the second output circuit 40. The typical multi-output power supply device using the SSPR circuit illustrated in FIG. 2 additionally includes the diode D2 for rectification. As such, power loss due to a voltage drop by the diode D2 occurs to decrease the efficiency of the circuit and increase the cost of the circuit.